The invention relates to a component made of an alloy of the AlMgSi type having a high capacity to absorb kinetic energy by plastic deformation.
Crash behaviour is becoming an increasingly important aspect of vehicle manufacture. This holds both for road transport and for rail transport.
The manufacturers of road and railway vehicles are increasingly dimensioning special components or complete units of the vehicle in such a way that, on collision, they are able to absorb as much energy as possible in order to reduce the risk of injury to the passengers.
Apart from the constructive aspects of these so called crash elements, the mechanical properties of the alloys employed and the joints are of decisive importance in this respect. Desired is the maximum possible absorption of energy before fracture. This can be achieved by a low ratio of yield strength to strength at rupture. An important material characteristic is also a high degree of elongation. The mechanical properties of joining areas such as weld seams should differ as little as possible from those of the parent metal. In the case of extrusions good elongation in the transverse direction is also of great importance.
Attention must also be given to the requirements made of the finished construction. For example specified values may be given for the construction with respect to strength, minimum elongation, corrosion resistance or other essential characteristics.
Among the aluminium alloys which are fabricated into crash elements today are in particular the standard alloys of the AlMgSi type. Alloys of this type, compared with others such as e.g. AlZnMg alloys, provide good preconditions in the form of elongation and formability for energy-absorbing parts. In spite of that, further optimisation of properties is desirable.
The alloy AA6005A employed at present for the construction of carriages gives rise to a series of problems in manufacture which are associated with the tendency for coarse grain recrystallisation. With a coarse grain microstructure it is difficult to achieve the given bending radii- the tendency for grain boundary opening being reinforced by welding. This leads to a large number of nonconformities in production. If this is to be avoided, then the parts must be manufactured such that the cross-section of the extrusion is mainly fibrous in grain structure. At present this is possible only using an alloy composition which leads to higher extrusion forces and significantly lower extrusion rates. This means that large penalties in productivity have to be accepted as a consequence.
Components employed as safety parts in automobile manufacture often do not have to reach the high strength values prescribed in railway carriage manufacture. On the other hand the extruded components employed in automobile manufacture often exhibit section wall thicknesses of the order of 1 mm or even less. Such low wall thicknesses cannot be extruded using alloys with too high strength, or at least not economically.
The object of the invention is to provide a material having especially good formability along with good mechanical properties in the component. The material should exhibit a strength level comparable to or slightly lower than that of the alloy AA6005A, but afford a higher degree of success in production and higher productivity.
The objective of the invention is achieved by way of the alloy containing in wt. %
and further elements each individually at most 0.05, in total at most 0.15 and the remainder aluminium.
The alloy according to the invention is with respect to strength and elongation much less quench sensitive than the alloy AA6005A and even at wall thicknesses of 6 mm exhibits fine grain structure throughout the whole of the cross-section. The alloy is therefore basically suitable for use in large extruded sections.
Silicon and magnesium are preferably restricted when the material is used in components with high strength requirements such as in railway carriages, these limitations in wt. % are as follows:
For components having lower strength requirements, such as extruded sections for automobile manufacture, and having partially low wall thickness of 1 mm or less, the following ranges in wt. % apply to silicon and magnesium:
The following preferred range of values holds for the other elements, other than silicon and magnesium, in the alloy according to the invention:
The use of the alloy composition according to the invention for the manufacture of components with a high capacity to absorb energy leads to a favourable microstructure in the component. Thexe2x80x94smallest possiblexe2x80x94grain size, obtained in order to improve the forming properties, is achieved by the alloy composition according to the invention.
A component with particularly good properties with respect to energy absorption along with good strength values, can be obtained using a special heat treatment. This comprises creating an underaged or partially hardened condition in the alloy i.e. the alloy is not hardened to give maximum strength. The underaged condition is obtained by heat treating (artificial age-hardening) in the range 120 to 170xc2x0 C. for an interval of 4 to 6 h. The desired degree of under-age-hardening may be determined by means of simple trials; the condition T64 is preferred.
A further preferred heat treatment, which in particular in the automobile industry can be combined with paint stoving, comprises heating between 190 and 230xc2x0 C. for an interval of 1 to 5 h. Such a treatment produces a slight overaging, the condition T72 being preferred.
The components according to the invention are, in the simplest case, extruded sections. Feasible, however, are also components which start from an extruded section as a preform and achieve their final shape as a result of pressure from within. According to a further version of the invention, the component may also be a forging.
A preferred use of the component according to the invention is for safety parts in automobile manufacture.
The advantages of the alloys according to the invention in their use for manufacturing crash elements and the special heat treatments for underaging or overaging are underlined by the following test results.